Device and method for sharpening rotary mower blades

ABSTRACT

A rotary mower blade sharpener device formed substantially of a resiliently deformable material and characterized as having abrasive properties and friction gripping properties on opposing surfaces thereof. The opposing surface having friction gripping properties is adapted, when laid upon the ground or other expansive surface, to retard rotation of the device when the blade to be sharpened is subject to momentary rotations in surface engagement with the opposing surface having abrasive properties. The relative rotation of the blade with respect to the surface of the device having abrasive properties effects sharpening of the blade.

United States Patent Warwick [54] DEVICE AND METHOD FOR SHARPENING ROTARY MOWER BLADES [72] Inventor: David W. Warwick, 3640 Upper Mountain Rd., Sanborn, NY. 14132 [22] Filed: Dec. 9, 1970 [21] Appl.No.: 96,565

[52] US. Cl ..51/250, 56/250, 51/288 [51] Int. Cl ..B24b 19/00 [58] Field of Search ..51/250, 149, 161, 288; 56/250 [56] References Cited UNITED STATES PATENTS Spear ..51/250 51 May30, 1972 3,315,452 4/1967 Rossi ..51/250 Primary Examiner-William R. Armstrong Attorney-Bean & Bean [57] ABSTRACT 9 Claims, 5 Drawing Figures I. 24 24 @W/fl/ 2i Patented May 30, 1972 3,665,658

INVENTOR. DA V/D l/V. W/lRW/CK ATTORNEYS DEVICE AND METHOD FOR SHARPENING ROTARY MOWER BLADES BACKGROUND OF THE INVENTION Conventionally, the blade or blades of rotary mowers or the like are sharpened by hand filing or machine grinding procedures, which are best performed with the blade removed from the mowenSharpening of a blade in this manner is of course time consuming with the result that the average home owner seldom maintains a proper cutting edge on his lawn mower blade.

Recognizing the difficulties associated with removing lawn mower blades for sharpening and appreciating the substantially increased efficiency of a mower when its blade is properly sharpened, various attempts have been made to provide lawn mowers with built-in blade sharpening devices. However, built-in blade sharpening devices have not come into wide use, due to the fact that they employ a rigid sharpening stone whose use requires that the stone be accurately positioned with respect to the cutting elements of the blade and that such cutting elements follow along identical tracts. This latter requirement is substantially impossible to achieve on commercially available equipment with the result that the sharpening stone is ofttimes shattered and/or the blade to be sharpened severely damaged. As will be clearly apparent, the shattering of the sharpening stone is a safety hazard, since pieces of the shattered sharpening stone are likely to be propelled from the mower at high velocity.

SUMMARY OF THE INVENTION The present invention relates to a lawn mower blade sharpening device and more particularly to a device which permits the blade to sharpen itself, while not being subject to the disadvantages of prior blade sharpening devices.

More specifically, the blade sharpening device fabricated in accordance with the preferred embodiment of the present invention is a disc shaped member formed substantially of a resiliently deformable material, such as for instance foam rubber and plastic foam. The diameter of opposing surfaces of the disc shaped member substantially corresponds to the diameter of an envelope defined by rotation of the blade to be sharpened, whereby permitting the device to be freely received within the protective shroud normally surrounding the blade.

In accordance with the preferred embodiment of the present invention, each of the opposing surfaces of the device are covered with abrasive material, such as is provided by a commercially available flexible abrasive sheet including fine grit sandpaper, garnet paper or emery cloth, and the device is formed with a centered opening, which extends through the opposing surfaces and is adapted to freely receive the stub end of the blade mounting drive shaft when the cutting elements of the blade to be sharpened are positioned in surface engagement with one or the other of such surfaces.

The thickness of the sharpening disc, when in non-compressed condition is chosen to be slightly in excess of one of the conventional blade height grass cutting settings normally provided for commercially available lawn mowers. Thus, when the sharpening device is laid on a supporting surface such as the ground or concrete driveway with one of its abrasive surfaces facing up, the blade is arranged at the height adjustment which corresponds to the thickness of the device and the lawn mower is set down over the device, the blade will be positioned in surface engagement with the upwardly facing abrasive surface of the device and the downwardly facing surface of the device will be maintained by the blade in frictional engagement with the supporting surface. Thereafter, momentary rotations of the blade, caused by either switching on or off an electric lawn mower or pulling the starter cord of a gasoline engine driven lawn mower with the spark plug disconnected, will effect sharpening of the blade cutting elements. In this respect, it will be understood that the friction gripping engagement of the downwardly facing abrasive surface of the device with the supporting surface will either prevent or retard rotations of the device with the blade.

Alternatively, the device may be provided with abrasive material on only one of its opposing surfaces. In this case, the other surface would have friction gripping properties imparted thereto by forming the device from a material having a high coefficient of friction, such as foam rubber, or by roughening or providing ribs projecting from a relatively slippery" plastic foam material, such as polyurethane foam.

DRAWINGS The nature and mode of the present invention will now be more fully described in the following detailed description taken with the accompanying drawings wherein:

FIG. 1 is a side elevational view of a conventional lawn mower positioned above a blade sharpening device formed in accordance with the present invention;

FIG. 2 is a fragmentary perspective view showing the rotor blade to be sharpened in surface engagement with the device of the present invention;

FIG. 3 is a fragmentary view showing a single cutting element in contact with the sharpening device of the present invention;

FIG. 4 is a sectional view illustrating a modified form of the sharpening device of the present invention; and

FIG. 5 is a sectional view illustrating yet a further modification of the sharpening device of the present invention.

DETAILED DESCRIPTION Now referring to the drawings in greater detail, 10 generally designates a blade sharpening device formed in accordance with a preferred embodiment of the present invention, and 12 generally designates a rotary type lawn mower of substantially standard construction.

For purposes of reference, mower I2 is shown in FIG. 1 as having a single mower blade 14 defining cutter elements 144 arranged adjacent opposite ends thereof. Blade 14 may be suitably fixed, as by lock nut 16, to mower drive shaft 18 for rotation about a generally vertically extending axis. A protective shroud, such as may be defined by mower housing peripheral wall 20 or cut grass directing inserts, not shown, is arranged outwardly of an envelope defined by rotation of blade 14. As is conventional, mower 12 is provided with suitable means, such as operating handles 22, for the purpose of adjusting lawn mower wheels 24 relative to the frame of the mower in order to vary the cutting height of blade 14, that is, the distance at which such blade is spaced from the ground, grass, driveway or other expansive supporting surface 26 on which wheels 24 are supported.

When lawn mower 12 is of the type employing a gasoline engine 28, momentary rotations of blade 14 may be effected by pulling starter cord 30 when the ignition wire to spark plug 34 is disconnected. When lawn mower 12 is of the type employing an electric motor, not shown, momentary rotation of blade 14 may be effected by merely turning on and off the electric motor control switch, also not shown. As will be more fully hereinafter described, one or more momentary rotations of blade 14 relative to sharpening device 10 are employed to effect self sharpening of its cutting elements 14a.

Now referring to FIGS. 1-3, it will be seen that blade sharpening device 10 includes a main body portion 40 formed of a resiliently formable material, such as for instance a sheet of rubber or plastic foam material. The foam material employed may be either of the open or closed cell variety, providing it possesses a fast recovery rate.

Opposing surfaces of body 40, which are generally designated as 42 and 44, are dimensioned to encompass an envelope defined by rotation of blade 14 when in juxtaposition, as illustrated in FIG. 1. When device 10 is fabricated for use with mowers having a single cutting blade and thus a substantially cylindrical protective shroud, body portion 40 is preferably shaped as a disc with the diameter of surfaces 42 and 44 at least equal to the diameter of the envelope developed by blade rotation, but less than the minimum transverse dimension of shroud in order to permit the device to freely pass inwardly of the shroud for engagement with the blade. Devices would of course be fabricated in a series of sizes accommodating for the range of sizes of commercially available mowers.

In the preferred embodiment of the present invention, which is illustrated in FIGS. 1-3, flexible abrasive sheets 46 and 48 are suitably attached as by adhesive bonding, to surfaces 42 and 44, respectively, and an opening 50 is arranged to extend centrally through body portion 40 and sheets 46 and 48. In the construction shown, opening 50 is dimensioned to freely receive lock nut 16 and the lower end of drive shaft 18 in order to prevent frictional contact thereof with device 10. For mower constructions employing blades supported for rotation about parallel drive shafts, body portion 40 and sheets 46 and 48 may have an oval plan view configuration and two through openings in order to insure continuous sharpening contact of blade cutting elements with the device, as well as permitting the device to be freely received within the blade enclosing shroud. In either construction, opening 50 performs the additional function of permitting the device to be hung up on a nail or other suitable support, so as to permit convenient storage thereof.

Sheets 46 and 48 may be selected from commercially available abrasive sheet materials including, for example, fine grit sand papers, garnet papers, emory cloths and fast cutting industrial cloths, whereas the sheet bonding adhesive may be any permanently flexible adhesive permitting flectures of the associated sheets and body portion without separation. This results in a construction, wherein the abrasive sheets become floating tables, which insure accurate following of the blade cutting elements, while preventing tearing of the foam support.

Alternately, it is anticipated that the abrasive sheets may be dispensed with and abrasive granules bonded directly to surfaces of the body portion, particularly when the latter is formed with a thin pliable skin, which tends to act as a substitute for the flexible sheet or cloth.

While the overall thickness of device 10 may vary, depending upon the construction of mower 12 for which it is designed for us, it is preferable to employ sheets of foam material having a thickness greater than about 1 inch in order to effectively isolate surfaces 42 and 44. However, since commercially available lawn mowers conventionally provide for a popular 2 inch grass cutting height setting for blade 14, body portion 40 is preferably formed from commercially available 2 inch sheet stock material in order not to require the average user to readjust the setting of the mower wheels each time the blade is to be sharpened. The effective non-compressed thickness of device 10 is slightly greater than the 2 inch blade setting or distance of the blade from the ground, due to the collective thicknesses of sheets 46 and 48, which will be less than about one-eighth inch and normally less than one-sixteenth inch.

Whenever it is desired to sharpen blade cutting elements 14a, it is merely necessary to lay device 10 on the ground or other generally flat supporting surface 26, as illustrated in FIG. 1, such as to position one of the abrasive sheets, such as sheet 48, in surface engagement therewith. Care should be exercised to insure that the device is laid down upon a relatively clean and dry surface in order to prevent damage and/or the accumulation of undesired coatings on the supporting abrasive sheet. Thereafter, mower 12 is moved over device 10 and blade 14 lowered into engagement with the upwardly facing abrasive sheet, such as sheet 46, as for example by pivoting mower 12 between its phantom and full line positions shown in FIG. 1. When blade 14 is thus positioned, it serves to very slightly deflect sheet 46, such as is shown in magnified form in FIG. 3, thereby positively maintaining sheet 48 in engagement with surface 26.

As will be apparent, the abrasive properties of downwardly facing abrasive sheet 48 effectively provide this surface of device 10 with friction gripping properties, such as to prevent or constrain rotation of the device relative to the supporting surface, whenever mower blade 14 is rotated while in engagement with upwardly facing abrasive sheet 46. Also, as will be apparent, the surface area of sheet 46 in engagement with the supporting surface is substantially greater than the surface area of sheet 46, which is in engagement with blade cutting elements 14a at any given time, whereby there is a greater tendency for the blade cutting elements to slide relative to sheet 46 than for sheet 48 to slide or rotate relative to the supporting surface.

It will be understood that during rotations of blade 14, body portion 40 serves to provide a substantially uniform bias for maintaining sheet 46 in sharpening engagement with blade cutting elements 140, while permitting flectures of sheet 46 accommodating for variations in or vertical displacements of the rotational tracks of the cutting elements. Moreover, body portion 40 is believed to permit slight rotational displacements of sheets 46 and 48, thus aiding in preventing the breaking of static frictional contact between sheet 48 and surface 26 at the initiation of blade rotation.

For mower blades in reasonably good cutting condition and with abrasive sheets 46 and 48 in new or only partially worn condition, sharpening of cutting elements 14a may be effected by only one or two momentary rotations of blade 14, as for instance produced by pulls on starter cord 30 when the ignition wire is disconnected from spark plug 34.

As by way of example, a cutting device demonstrating a long service life has been formed by cutting a disc from a 2 inch thick sheet of elastic polyurethane foam material having a density of about 2 lbs. per cubic foot, the foam material demonstrating a relatively fast recovery rate and about 50 percent compressibility under an applied load of about 50 psi. This type of foam material is a partially closed cell in structure and commercially available for use as a package cushioning material. Two sheets of a commercially available fast cut drum paper sold under the Trademark Aloxite", manufactured and sold by the Carborendum Co., were cut to size and bonded to opposite surfaces of the disc by standard type of permanently flexible rubber cement. The paper is believed to have a lb. backing paper to which aluminum oxide particles of about 40 micron average size are bonded. The thus constructed sharpening device was employed in over 30 different occasions to satisfactorily sharpen the cutting elements of a rotary mower.

A sharpening device was also fabricated by using foam rubber to form the body portion, the latter having the above described fast cut drum paper bonded thereto. This construction was also effectively employed in the sharpening of blades with little noticeable difference.

Of course, the efficiency cost and useable life expectancy of the sharpening device is expected to vary substantially depending upon the type of foam material and/or abrasive sheet employed.

FIG. 4 illustrates a modified form of the sharpening device of the present invention generally designated as 10'. Device 10' differs from the construction described hereinabove in that a pair of abrasive sheets, such as 46, is affixed in juxtaposition to only one surface of body portion 40; the body portion being formed from a material having a relatively high coefficient friction, such as foam rubber, in order to prevent or constrain slippage of device 10' relative to the ground during sharpening rotation of the mower blades relative to sheets 46'. Sheets 46' may be removably fixed together by any suitable adhesive, which permits the top sheet, when worn out, to be pulled or peeled off the lower sheet without damage to the latter. Also, in this construction, opening 50' is only of a depth sufficient to freely accommodate any blade attaching device or grass guard depending below the mower blade, so as to permit the entire lower surface of body surface 40 to be available for the purposes of constraining rotation.

FIG. 5 illustrates a further modification of the sharpening device of the present invention, which is generally designated as 10'. In device 10" the lower surface 44" of the body portion 40" is roughened, as by providing radially extending ribs 60 in order to increase the frictional gripping properties of such surface. This construction is particularly desirable wherein body portion 40" is formed from a relatively slippery plastic foam material and/or from a foam material having a relatively smooth skin formed during the molding operation. Further, device is shown as having a relatively large diameter through opening 50" in order to accommodate for mower blade constructions of the type wherein the radial dimension of the blade cutting elements is relatively short, such as for instance wherein two or more blades are mounted to project radially from a circular blade support which is in turn secured to the mower drive shaft. As in the case of the modification of FIG. 4, device 10 would preferably be formed with two or more abrasive sheets 46 I claim: 1. A method for sharpening the blade of a rotary mower, wherein said blade is supported for rotation about the axis of a vertically extending drive shaft, including the steps of:

providing a sharpening device formed substantially of a resiliently deformable material wherein opposing surfaces of said device are dimensioned to encompass an envelope defined by rotation of said blade when in juxtaposition, at least one of said opposing surfaces having abrasive properties and the other of said opposing surfaces having friction gripping properties; laying said device on a substantially horizontal generally planer supporting surface with said opposing surface having abrasive properties facing upwardly and said opposing surface having frictional gripping properties facing downwardly in engagement with said supporting surface;

lowering said blade towards said supporting surface so as to place said blade in surface engagement with said opposing surface having abrasive properties and thereby maintain said opposing surface having friction gripping properties in engagement with said supporting surface; and

effecting at least one momentary rotation of said blade relative to said opposing surface having abrasive properties while maintaining the plane of said envelope substantially parallel to the last said surface, thereby to effect sharpening of said blade.

2. A method for sharpening the blade of a wheel supported rotary mower of the type wherein said blade is supported for rotation about the axis of a vertically extending drive shaft and is positioned within a protective shroud arranged outwardly of an envelope defined by said blade upon rotation thereof, said mower supporting wheels being mounted on said mower to position said blade in at least one cutting distance from an expansive mower supporting surface, including the steps of:

providing a generally disc shaped sharpening device formed substantially of a resiliently deformable material, wherein the diameter of opposing surfaces of said device substantially corresponds to the diameter of said envelope to permit said device to be freely received inwardly of said protective shroud, at least one of said opposing surfaces having abrasive properties and the other of said opposing surfaces having friction gripping properties, and the thickness of said device as measured between said opposing surfaces being of a dimension slightly greater than said one distance; laying said device on said supporting surface with said opposing surface having friction gripping properties in contact with said supporting surface;

lowering said mower to position said wheels in engagement with said supporting surface with said device arranged within said shroud and said blade arranged in surface contact with said opposing surface having abrasive properties to thereby maintain said opposing surface having friction gripping properties in frictional surface engagement with said supporting surface; and

effecting one or more momentary rotations of said blade sufficient to effect sharpening of said blade by said device. 3. A device for sharpening the blade of a rotary mower of the type wherein said blade is supported for rotation about the axis of a verticallyextending drive shaft and positioned within a protective shroud arranged outwardly of the envelope defined by said blade upon rotation thereof, and wherein said blade is arrangeable in at least one cutting distance from a supporting surface for said mower, said device comprising in combination:

a body portion formed substantially of a resiliently compressible material wherein opposing surfaces of said body portion are dimensioned to encompass said envelope when in juxtaposition and to permit said body portion to be freely received inwardly of said protective shroud, at least one of said opposing surfaces having abrasive properties and the other of said opposing surfaces having friction gripping properties, the distance between said opposing surfaces being such that when said opposing surface of said body portion having friction gripping properties is laid face down on said supporting surface and said blade is lowered towards said device and arranged at said cutting distance, said device is arranged within said shroud and said blade is positioned in contact with said opposing surface having abrasive properties, whereby said blade may be sharpened by one or more momentary rotations thereof relative to said device while in contact with said opposing surface having abrasive properties, rotation of said device with said blade during momentary rotations thereof being constrained by frictional engagement of said device with said supporting surface.

4. A blade sharpening device according to claim 3, wherein said body portion is of disc shaped configuration and an opening in said body portion opens at least centrally through said opposing surface having abrasive properties.

5. A blade sharpening device according to claim 3, wherein said opposing surface having abrasive properties is characterized as including at least one sheet of flexible abrasive material adhesively bonded to said body portion.

6. A blade sharpening device according to claim 5, wherein at least two of said sheets of material are provided, said sheets of material being removably joined together in juxtaposition.

7. A blade sharpening device according to claim 3, wherein said body portion is formed from foam rubber, an uncovered surface of said foam rubber providing the friction gripping properties of said other opposing surface.

8. A blade sharpening device according to claim 3, wherein said abrasive and friction gripping properties of said opposing surfaces are defined by sheets of flexible abrasive material bonded at least one to each of said opposing surfaces.

9. A blade sharpening device according to claim 8, wherein said body portion is of disc shaped configuration, and an opening in said body portion opens centrally through each of said opposing surfaces. 

1. A method for sharpening the blade of a rotary mower, wherein said blade is supported for rotation about the axis of a vertically extending drive shaft, including the steps of: providing a sharpening device formed substantially of a resiliently deformable material wherein opposing surfaces of said device are dimensioned to encompass an envelope defined by rotation of said blade when in juxtaposition, at least one of said opposing surfaces having abrasive properties and the other of said opposing surfaces having friction gripping properties; laying said device on a substantially horizontal generally planer supporting surface with said opposing surface having abrasive properties facing upwardly and said opposing surface having frictional gripping properties facing downwardly in engagement with said supporting surface; lowering said blade towards said supporting surface so as to place said blade in surface engagement with said opposing surface having abrasive properties and thereby maintain said opposing surface having friction gripping properties in engagement with said supporting surface; and effecting at least one momentary rotation of said blade relative to said opposing surface having abrasive properties while maintaining the plane of said envelope substantially parallel to the last said surface, thereby to effect sharpening of said blade.
 2. A method for sharpening the blade of a wheel supported rotary mower of the type whereiN said blade is supported for rotation about the axis of a vertically extending drive shaft and is positioned within a protective shroud arranged outwardly of an envelope defined by said blade upon rotation thereof, said mower supporting wheels being mounted on said mower to position said blade in at least one cutting distance from an expansive mower supporting surface, including the steps of: providing a generally disc shaped sharpening device formed substantially of a resiliently deformable material, wherein the diameter of opposing surfaces of said device substantially corresponds to the diameter of said envelope to permit said device to be freely received inwardly of said protective shroud, at least one of said opposing surfaces having abrasive properties and the other of said opposing surfaces having friction gripping properties, and the thickness of said device as measured between said opposing surfaces being of a dimension slightly greater than said one distance; laying said device on said supporting surface with said opposing surface having friction gripping properties in contact with said supporting surface; lowering said mower to position said wheels in engagement with said supporting surface with said device arranged within said shroud and said blade arranged in surface contact with said opposing surface having abrasive properties to thereby maintain said opposing surface having friction gripping properties in frictional surface engagement with said supporting surface; and effecting one or more momentary rotations of said blade sufficient to effect sharpening of said blade by said device.
 3. A device for sharpening the blade of a rotary mower of the type wherein said blade is supported for rotation about the axis of a vertically extending drive shaft and positioned within a protective shroud arranged outwardly of the envelope defined by said blade upon rotation thereof, and wherein said blade is arrangeable in at least one cutting distance from a supporting surface for said mower, said device comprising in combination: a body portion formed substantially of a resiliently compressible material wherein opposing surfaces of said body portion are dimensioned to encompass said envelope when in juxtaposition and to permit said body portion to be freely received inwardly of said protective shroud, at least one of said opposing surfaces having abrasive properties and the other of said opposing surfaces having friction gripping properties, the distance between said opposing surfaces being such that when said opposing surface of said body portion having friction gripping properties is laid face down on said supporting surface and said blade is lowered towards said device and arranged at said cutting distance, said device is arranged within said shroud and said blade is positioned in contact with said opposing surface having abrasive properties, whereby said blade may be sharpened by one or more momentary rotations thereof relative to said device while in contact with said opposing surface having abrasive properties, rotation of said device with said blade during momentary rotations thereof being constrained by frictional engagement of said device with said supporting surface.
 4. A blade sharpening device according to claim 3, wherein said body portion is of disc shaped configuration and an opening in said body portion opens at least centrally through said opposing surface having abrasive properties.
 5. A blade sharpening device according to claim 3, wherein said opposing surface having abrasive properties is characterized as including at least one sheet of flexible abrasive material adhesively bonded to said body portion.
 6. A blade sharpening device according to claim 5, wherein at least two of said sheets of material are provided, said sheets of material being removably joined together in juxtaposition.
 7. A blade sharpening device according to claim 3, wherein said body portion is formed from foam rubber, an uncovered surface of said foam rubbEr providing the friction gripping properties of said other opposing surface.
 8. A blade sharpening device according to claim 3, wherein said abrasive and friction gripping properties of said opposing surfaces are defined by sheets of flexible abrasive material bonded at least one to each of said opposing surfaces.
 9. A blade sharpening device according to claim 8, wherein said body portion is of disc shaped configuration, and an opening in said body portion opens centrally through each of said opposing surfaces. 